Stiff-stilbene derivatives as new bright fluorophores with aggregation-induced emission

Stiff-stilbene derivatives have been widely explored as molecular rotors, molecular force probes and optical switches with excellent performance. However, their function as fluorophores is poorly understood. In the present work, we design three stiffstilbene derivatives and study their photophysical properties. These compounds exhibit very weak emission in solution but significantly enhanced monomer emission in viscous solvent, bright excimer emission in aggregates and at solid state. Detailed spectroscopic studies, single crystal structural analysis, powder X-ray diffraction(XRD) as well as effects of substituents have been carefully examined. They provide direct evidence that intermolecular interactions and molecular packing, which can restrict bond vibration and rotation, are responsible for the bright aggregation-induced emission.     

Fluorescent probe as reporter on the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes

To get some information on the aggregation behaviors of the products derived from different organotrialkoxysilanes, the hydrolysis-condensation processes of some organotrialkoxysilanes have been examined by means of pyrene as fluorescent probe. The organotrialkoxysilanes used in the research were n-octadecyltri-methoxysilane (ODTMS), n-octyltrimethoxysilane (OTMS), 3-glycidoxypropyltrimethoxysilane (GTMS), 3-methacryloxypropyltrimethoxysilane (MAPTMS), and propyltrimethoxy-silane (PTMS). The results show that pyrene as fluorescence probe can respond sensitively not only to the organization state of the hydrolysates but also to the change in the organization state during the condensation process. The organization states during the hydrolysis and condensation can be explained in terms of structures of the products. In the initial stage, the silanols with long organic chains are amphiphilic molecules, and such nature of the silanols can be compared to that of a surfactant. Therefore, the excimer emission of pyrene is extremely obvious because of such silanols being prone to form aggregates. In the case of silanols having short alkyl groups or epoxy groups, these silanols homogenously disperse in solution, which results in the appearance of an only monomer emission of pyrene. In the late stage, the fluorescence behavior of pyrene is also sensitive to structural evolution of the silicates. The fluorescence spectra of pyrene during the condensation of the silanols with short alkyl groups or epoxy groups are almost in silence, indicating that the condensation products, with a low condensation degree, homogeneously disperse in solution. For the silanols with long hydrophobic substituents in different lengths, the changes in fluorescence spectra of pyrene during the condensation are varied. Commonly, the excimer emission is noticeable, implying that the condensation products with high condensation degree inhomogenously disperse in solution. However, the relative excimer/monomer fluorescence intensity is alkyl chain-length dependent. The longer alkyl chains in the condensation products result in the appearance of the obvious excimer emission. These phenomena imply that the condensation degree of the products increases with the length of the alkyl chains. Additionally, the distorted spectrum of pyrene appears in the case of the organotrialkoxysilanes with side chain substituent, illustrating that the steric hindrance between the substituents can be monitored by fluorescence of pyrene. All these results are verified by the fluorescence-quenching measurements. The approach in the present study gives new insights into the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes and emphasizes the influence of the self-assembling behavior.     

Structural Engineering of Luminogens with High Emission Efficiency Both in Solution and in the Solid State

Developing molecules with high emission efficiency both in solution and the solid state is still a great challenge, since most organic luminogens are either aggregation‐caused quenching or aggregation‐induced emission molecules. This dilemma was overcome by integrating planar and distorted structures with long alkyl side chains to achieve DAπAD type emitters. A linear diphenyl–diacetylene core and the charge transfer effect ensure considerable planarity of these molecules in the excited state, allowing strong emission in dilute solution (quantum yield up to 98.2 %). On the other hand, intermolecular interactions of two distorted cyanostilbene units restrict molecular vibration and rotation, and long alkyl chains reduce the quenching effect of the π–π stacking to the excimer, eventually leading to strong emission in the solid state (quantum yield up to 60.7 %).     

MOLECULAR CONFORMATION OF IONIC SURFACTANTS IN AQUEOUS SOLUTION

The molecular conformation of ionic surfactant in aqueous solution is investigated withfluorescent probes which are intrinsic insurfactant molecules or externally introduced. Quench-lng or pyrene monomer fluorescence by alkyltriphenylphosphonium or N-alkylpyridiniumobeys Stern-Voimer equation, being diffusi6n-controlled dynamic quenching, but the behaviorof quenching with different lengths of alkyl chain is "abnormal", i.e. the longer the chain,the greater the quenching rate constant. The pyrene excimer fluorescence is observed in theaqueous solution of cetyltrimethylammonium bromide (CTMAB), and the inhibition (for cationicquenchers) and promotion (for anionic quenchers) effects of CTMAB on the quenching ofpyrene fluorescence are also observed. The self-coiling conformation of ionic surfactantmolecules in aqueous solution is proposed to be responsible for these observations and theconformation to be dynamic.     

Introduction of Ferrocene as a Facilitator for the Construction of Supramolecular Polymers

Proper monomer design is the key to enhancing the strength of noncovalent interactions between the molecules toward the efficient formation of supramolecular polymers (SPs). We have designed and synthesized 1,n′-disubstituted ferrocene-azobenzene-long alkyl chains, Fc(CONH-Azo-TDP)₂, to afford SPs with a high probability. The design exploits the ‘‘molecular ball-bearing’’ property of the ferrocene core, which allows two azobenzene arms to rotate in the planes of cyclopentadienyl rings, generating the most suitable molecular conformation required for SP formation. This ferrocene monomer formed a supergel consisting of SPs supported by strong intermolecular (H-bonding and π-π stacking) interactions and higher enthalpy gain than the reference molecules, where the central ferrocene core was replaced by flexible aliphatic as well as rigid benzene linkers. The molecular conformation involved in SPs, the strength of noncovalent interactions, and the process of supramolecular polymerization were investigated through NMR, UV-Vis, XRD and TEM studies. The results demonstrate that ferrocene may act as a good modulator for constructing efficient SPs.     

疏水作用对光化学和光物理过程的影响 I: 不良溶剂中长链β-萘甲酸烷基酯的簇集和激基缔合物的形成

研究了不同链长的β-萘甲酸烷基酯(An)在乙二醇-水(EG-H2O)和二甲基砜-水(DMSO-H2O)混合溶剂中的荧光光谱, 以及添加物(无机盐、长链饱和烷烃、糖淀粉)对An荧光的影响. 长链An在混合溶剂中很容易形成激基缔合物, 表明疏水作用促使长链分子相互簇集. 测定了不同链长的分子发生簇集的临界浓度和临界溶剂组成. 分别添加长链烷烃和糖淀粉都能引起激基缔合物的荧光强度减弱和单体荧光强度的增强, 表明An和长链烷烃共簇集, 与糖淀粉形成包结物. 研究了在簇集体中An形成激基缔合物的动力学和热力学, 测定了激基缔合物形成和解离速率常数、活化能和热焓的变化. 证明了簇集体中基态发色基团之间并不具有激基缔合物的构型, 在一定温度下, 簇集体会发生相变.     

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.     

芘在改性硅胶上的光物理研究——以芘为探针研究改性硅胶的表面特性

研究了芘在正十碳烷氧基和三甲基硅氧基改性硅腔表面上的荧光光谱和寿命。在这二种硅胶上,激基缔合物是由基态聚积体直接受光和受激发的单分子和基态的单分子所形成、在硅胶≡Si—O—C_(10)H_(21)-n上比在硅胶≡Si—O—SiMe_3上所形成的聚积体较少。化学改性与物理改性相结合可使芘在较大的浓度范围内主要以单分子分散。激基缔合物的形成主要是由动力学过程所控制。研究了温度对芘的荧光光谱和寿命的影响。激基缔合物形成过程的活化能约为7kcal mol~(-1)。讨论了环境对单分子荧光光谱结构的影响。     

Spectroscopy and photophysics of methylphenylsiloxaneand diphenylsiloxane-based molecules and polymers

Spectroscopy and photophysics of various types of methylphenylsiloxane- and diphenylsiloxane-based oligomers and polymers are reviewed. The molecules treated here include homopolymers such as poly(methylphenylsiloxane) and copolymers such as poly(dimethylsiloxane-codiphenylsiloxane) as well as related oligomers or molecules such as diphenyltetramethyldisiloxane. These polymers and oligomers normally exhibit monomer fluorescence in fluid solution at temperatures near room temperature, and the monomer fluorescence and phosphorescence in rigid matrices at 77 K. In addition to these emissions, the excimer fluorescence is often observed depending on the molecular structure of the siloxanes. These emission properties are rationalized based on the molecular structure and kinetics of the excimer formation processes as well as on the flexibility of the Si-O-Si bonds.     

Hydrophobic effects on photophysical and photochemical processes: 1. Aggregation and excimer formation of alkyl β-naphthoates with long chain in poor solvents

The fluorescence spectra of alkyl β-naphthoates with various chain lengths (A_n) in DMSO-H₂O and ethylene glycol-water (EG-H₂O) mixtures were studied. The β-naphthoates with short chain show monomer fluorescence only in both solvent mixtures, while fluorescence spectra of long chain alkyl β-naphthoates are dominated by excimer emission. Addition of long chain hydrocarbon or amylose resulted in the reduction of excimer emission and enhancement of monomer fluorescence. All these experimental results supported intermolecular aggregation of long chain alkyl β-naphthoates in poor solvents. The kinetic parameters of the formation and dissociation of excimer as well as fluorescence polarization in aggregates were measured. These data provided an insight into the characteristics of aggregates.     

Electric field effects on absorption and fluorescence spectra of trimethylsilyl- and trimethylsilylethynyl-substituted compounds of pyrene in a PMMA film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.     

Luminescent Radical‐Excimer: Excited‐State Dynamics of Luminescent Radicals in Doped Host Crystals

The excited‐state dynamics of the photostable luminescent organic radical (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl (PyBTM) doped in a host crystal was investigated by using optically detected magnetic resonance (ODMR) and time‐resolved emission spectroscopies. In the radical system, the unpaired electron can be used as the probe for studying the electronic state and its dynamics. The mixed crystal with a high concentration of the radical showed excimer emission, together with the monomer emission. The ODMR signals were observed with opposite signs for monitoring the monomer and the excimer emissions. Based on their temperature and concentration dependencies, the excited‐state dynamics on the doped crystal and the mechanism of the excimer formation and the ODMR signal generation are discussed with the help of the quantum mechanical simulation of the excited‐state spin dynamics. The initial process of excimer formation has been clarified for the first time from the viewpoint of the spin‐dynamics.     

Probing the conformation of polyelectrolytes in mesoporous silica spheres

We report a fluorescence-based approach to probing the conformation of a macromolecule, poly(allylamine hydrochloride) (PAH), in bimodal mesoporous silica (BMS) particles. The method involves monitoring the fluorescent properties of the probe, 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (4-PSA), upon electrostatic binding to PAH molecules adsorbed in the nanopores of the BMS particles. PAH infiltration into the BMS particles, quantified by thermogravimetric analysis and visualized by confocal laser scanning microscopy, was examined as a function of PAH adsorption time, PAH molecular weight, and the sodium chloride (NaCl) concentration and pH of the PAH adsorption solution. The conformation of PAH molecules in the nanopores was investigated by incubating the PAH-loaded BMS particles in 4-PSA and using the ratio of the excimer to monomer emission intensity to discern differences in the PAH conformation in the nanopores. Control experiments involving nonporous silica (NS) particles were also conducted to determine the extent to which the nanopores within the BMS particles influence the degree of PAH adsorption and the conformation of the adsorbed PAH molecules. The data indicate that PAH molecules adsorbed in the nanopores adopt a more coiled conformation than PAH molecules adsorbed on NS particles over a wide range of conditions. Further, the conformation of PAH molecules in the nanopores can be tuned by adjusting the NaCl concentration and/or pH of the PAH adsorption solution. 4-PSA titration experiments revealed that at saturation binding there are ca. 3.8 PAH monomer units per 4-PSA molecule. This study provides insights into macromolecule infiltration and conformation in nanopores, which are important for the application of mesoporous materials in the fields of adsorption/immobilization, catalysis, delivery, sensing, separations, and synthesis.     

Effect of temperature and surfactant structure on the microviscosity at the micelle-water interface: isomeric alkylbenzenesulfonates used as their own probe

We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xphiC16, where x=4-6 and indicates the position of the benzene ring [phi] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure. etaint/tau ratios, where etaint is the microviscosity of the benzene environment in micelles and tau its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of etaint. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60 degrees C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, showing that when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (IE/IM and p) yield the same information, namely, that the benzene moiety in all xphiC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (approximately 8-60 degrees C).     

D-π-A分子H-聚集体的电子光谱

合成了具有强化给体和强电子受体的D-π-A分子5-(4-N,N-二甲氨基苯乙烯基)-(1H,3H)-2,4,6-嘧啶三酮(AB1)和5-(4-N,N-双十八烷基氨基苯乙烯基)-(1H,3H)-2,4,6-嘧啶三酮(AB18),并对其电子光谱进行了研究,AB18在环 聚集体的吸收光谱及AB1和AP18的表面光电压谱在450nm附近出现H-聚集体的谱带,比其在氯仿溶液中单体的ICT带蓝移了30nm左右,AB1和AB18固体及其高浓度的氯仿溶液分别在620nm和610nm附近出现来源于H-聚集体的激基缔合物的荧光,620nm带比单体红移3000cm^01 左右,还探讨具有强电子给体和受体的D-π-A分子间π-π相互作用.     

Double Naphthalene‐Tagged Cyclodextrin‐Peptide Capable of Exhibiting Guest‐Induced Naphthalene Excimer Fluorescence

A cyclodextrin‐peptide hybrid (17NNβ) bearing two naphthalene units in the peptide side chain has been designed and synthesized as a novel chemosensor molecule. Circular dichroism study of the compound revealed that the peptide has α‐helix structure with a helix content of 41%. The peptide revealed both monomer and excimer emission and the intensity of the excimer emission increased while that of the monomer emission decreased upon addition of the guest compound. This behavior was observed for various guest molecules, suggesting that the system can be used for detecting molecules in aqueous solution.     

四(4,4',4'',4'''-N,N-二氨基)四苯乙烯光致发光与分子的构象效应

利用还原偶联方法合成出新化合物四(4,4',4',4''-N,N-二氨基)四苯乙烯（ TDETE）。通过测定该化合物在溶液、掺杂聚合物中及晶体粉末的稳态-瞬态荧光光 谱、荧光量子产率和辐射衰变速率常数等。讨论了分子的构象效应等因素对TDETE 光致发光行为的影响。在一定浓度下TDETE溶液存在着三个发光带,分别为全扭曲 构象分子（位于345nm附近的发光I带）、半扭曲构象分子（位于430nm附近的发光 II带）和激基缔合物（530发光III带）的辐射衰变所致。在聚合物（PMMA）中,一 方面由于分子单键的自由旋转扭曲受到遏制,表现为II带的辐射衰变速率常数(kf 值)增大、同时非辐射衰变速率常数knf值减小;另一方面,TDETE分子之间相互作 用得到加强而有利一缔合物形成,结果,使发光II带和III带合二为一出现强而宽 的发射峰,荧光量子产率从溶液中的0.055提高到0.855。此外,在PMM介制裁中观 测到TDETE分子聚集体在626nm处的发光带（IV）,数粉末态中聚集体IV带的强度骤 增,峰值波长红移至650nm。     

Pyrene excimer signaling molecular beacons for probing nucleic acids

Molecular beacon DNA probes, containing 1-4 pyrene monomers on the 5' end and the quencher DABCYL on the 3' end, were engineered and employed for real-time probing of DNA sequences. In the absence of a target sequence, the multiple-pyrene labeled molecular beacons (MBs) assumed a stem-closed conformation resulting in quenching of the pyrene excimer fluorescence. In the presence of target, the beacons switched to a stem-open conformation, which separated the pyrene label from the quencher molecule and generated an excimer emission signal proportional to the target concentration. Steady-state fluorescence assays resulted in a subnanomolar limit of detection in buffer, whereas time-resolved signaling enabled low-nanomolar target detection in cell-growth media. It was found that the excimer emission intensity could be scaled by increasing the number of pyrene monomers conjugated to the 5' terminal. Each additional pyrene monomer resulted in substantial increases in the excimer emission intensities, quantum yields, and excited-state lifetimes of the hybridized MBs. The long fluorescence lifetime ( approximately 40 ns), large Stokes shift (130 nm), and tunable intensity of the excimer make this multiple-pyrene moiety a useful alternative to traditional fluorophore labeling in nucleic acid probes.     

Intramolecular excimer emission of triply bridged [3.3.N](3,6,9)carbazolophanes

The photophysical properties of a series of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n = 3, 4, 5, 6) were studied as a model compound for a fully overlapped carbazole excimer. In these [3.3.n]Cz molecules, a plane angle of the two carbazole moieties changed systematically from nearly parallel to oblique, with increases in the length of the methylene chain n bridging at the 9-position of each carbazole ring. Absorption bands of [3.3.n]Cz showed the blue-shift and the splitting for (1)L(a) <-- (1)A and (1)L(b) <-- (1)A transition bands of the reference carbazole monomer, respectively. These spectral changes in [3.3.n]Cz were explained by Kasha's molecular exciton theory with the distance r and dihedral angle theta between the carbazole moieties in the ground state. In both liquid and glass matrixes, [3.3.n]Cz showed intramolecular excimer emission. The emission peak wavelength changed from 409 nm (n = 6) to 480 nm (n = 3) depending on r in the ground state. The dependence of the peak wavelength on r clearly showed that relative configurations of carbazole moieties in the ground state were preserved even in the excimer states. The smaller radiative rate of the excimer emission than the reference monomer was explained by the dimer symmetry of [3.3.n]Cz.     

Violet-to-blue tunable emission of aryl-substituted dispirofluorene-indenofluorene isomers by conformationally-controllable intramolecular excimer formation

Two series of DiSpiroFluorene-IndenoFluorene (DSF-IF) positional isomers, namely dispiro[2,7-diarylfluorene-9',6,9',12-indeno[1,2-b]fluorenes], (1,2-b)-DSF-IFs 1 and dispiro[2,7-diarylfluorene-9',6,9',12-indeno[2,1-a]fluorenes], (2,1-a)-DSF-IFs 2 have been synthesized. These violet-to-blue fluorescent emitters possess a 3π-2spiro architecture, which combines via two spiro links two different indenofluorene cores, that is, (1,2-b)-IF or (2,1-a)-IF and 2,7-substituted-diaryl-fluorene units. Due to their different geometric profiles, the two families of positional isomers present drastically different properties. The marked difference observed between the properties of (1,2-b)-DSF-IF (1) and (2,1-a)-DSF-IF (2) is discussed in terms of intramolecular π-π interactions occurring in (2,1-a)-DSF-IF (2) leading to conformationally-controllable intramolecular excimer formation. Indeed, the original geometry of the (2,1-a)-DSF-IF (2) family, with face-to-face "aryl-fluorene-aryl" moieties, leads to remarkable excimer emission through intramolecular π-π interactions in the excited state. Furthermore, the emission wavelengths can be gradually modulated by the control of the steric hindrance between the adjacent substituted phenyl rings. Thus, through a comparative and detailed study of the (1)H NMR, electrochemical and photophysical properties of DSF-IFs 1 and 2, we have evidenced the intramolecular π-π interactions occurring between the two "aryl-fluorene-aryl" moieties in the ground state and in the excited state. These properties have been finally correlated to the spectacular conformational change modeled by density functional theory (DFT) calculations. Indeed, the two "aryl-fluorene-aryl" moieties switch from a staggered conformation in the ground state to an eclipsed conformation in the first excited state.